In typical olefin plants, such as illustrated in U.S. Pat. No. 7,223,895, there is a front-end demethanizer for the removal of methane and hydrogen followed by a deethanizer for the removal of ethane, ethylene and C2 acetylene. The bottoms from this deethanizer tower consist of a mixture of compounds, including olefins, ranging in carbon number from C3 to C6. This mixture may be separated into different carbon numbers, typically by fractionation. Once separated, the C3-C6 olefins may undergo isomerization and metathesis to produce a desired product.
The metathesis catalysts and the double bond isomerization catalysts used are generally quite sensitive to poisons. Poisons include water, CO2, and oxygenates, such as ethers and alcohols. It is common practice to employ guard beds upstream of the isomerization/metathesis reaction system to insure the removal of these poisons. In practice these guard beds are either directly before the metathesis reaction system or further upstream.
Double-bond isomerization catalysts, such as magnesium oxide, are currently commercially used in the form of tablets having an effective diameter of about 5 mm. As used herein, effective diameter refers to the diameter that non-spherical shaped particles would have if it were molded into a sphere. These tablets exhibit good isomerization activity when processing butenes alone. However, such tablets exhibit activity for isomerization of 1-butene to 2-butene only for a short time in the presence of poisons. Further, their performance is progressively worse as the number of reaction cycles increase. After several regeneration/reaction cycles, their activity for isomerization is low. This performance shortfall may lead to a rapid buildup of 1-butene in the system over time, limiting reactor performance by hydraulically limiting the recycle, and limiting the overall conversion of butenes to propylene or other end products that can be obtained economically. A similar loss of activity is experienced when operating these catalysts as double bond isomerization catalysts alone for the production of the terminal olefin from the interior olefin.
Some attempts have been made to improve the performance of magnesium oxide catalysts. For example, U.S. Pat. No. 6,875,901 discloses improvements to the deactivation rate of magnesium oxide isomerization catalysts by limiting certain impurities, such as phosphorous, sulfur, transition metals, etc. Deactivation in the presence of oxygenates, however, remains problematic.